Many liquids have dissolved within them a gas, either of the same chemical composition as the liquid itself or of a different chemical composition. When the pressure of such a gas containing liquid is reduced below a certain pressure, known as the saturation pressure, the gas dissolved in the liquid evolves out of solution to form a distinct gas phase separate from the liquid itself. When this process occurs within a sealed vessel the gas phase evolves from the liquid to form a distinct gas phase above the liquid.
A common example of this process occurs in carbonated drinks. The carbonated drinks container is pressurised when the container is sealed. Once a consumer purchases the carbonated drink and opens the container the pressure of the carbonated liquid is reduced below the saturation pressure. This results in the dissolved gas, carbon dioxide, evolving from the liquid and the recognised hiss of the gas escaping from the container as it opens can be heard.
Although the dissolved gas escaping from the pressurised container is not of particular concern in the carbonated drinks industry, it is of major concern in other industries and in particular the upstream oil industry. When crude oil is extracted from underground wells, the oil contains a significant amount of dissolved natural gas. Natural gas is highly flammable and thus, for safety reasons, it is necessary that for storage and transportation to remove the dissolved natural gas from the crude oil. The gas also needs to be removed so that the crude oil can be transported via pipelines. If the gas was not removed then, as the oil moves along the pipeline, the pressure would inevitably drop within the pipeline at some stage, resulting in the gas evolving from solution and in doing so it would block the oil flow. The natural gas is typically removed from the crude oil by reducing the pressure of the crude oil, which when extracted from underground has a very high pressure, such that the natural gas evolves out of solution. Typically the pressure of the crude oil is reduced in stages over a period of time, until the crude oil reaches atmospheric pressure.
Once the crude oil has been de-pressurised to remove the natural gas dissolved therein, it is then necessary to re-pressurise the crude oil for transportation. Since crude oil is generally transported through pipelines, in order to cover any significant distance by such a pipeline the crude oil needs to be re-pressurised to a pressure of several hundred PSI. This re-pressurisation may be carried out using electrically or mechanically powered pumps.
This de-pressurising and re-pressurising of the crude oil involves a significant amount of energy, resulting in a high cost overall both financially and environmentally in terms of wasted resources.
It is an object of the present invention to overcome or mitigate some of the problems above.